1. Field of the Invention.
The present invention relates to an apparatus and method for shaping and concentrating broad area laser beams and beams from broad area diode lasers, diode laser bars and/or diode laser arrays into narrow overlapping beams for coupling, for example, to an optical-fiber amplifier, to a solid-state laser, etc.
2. Relevant Technology and Prior Art.
A laser is a device which utilizes the transitions between energy levels of atoms or molecules to amplify or generate light. When an electron makes a transition from a higher energy level to a lower energy level, a photon, the elementary quantity of radiant energy, is emitted. In what is referred to as "stimulated emission", an incoming photon stimulates an electron to change energy levels, which amplifies the number of exiting photons. In fact, this is the origin of the term laser: light amplification by the stimulated emission of radiation. The emitted photon travels in the same direction and is in the same phase as the incoming photon.
One example of a conventional laser is the semiconductor diode laser. Semiconductor lasers are particularly useful for several reasons: they are capable of generating coherent radiation in the wavelength range which is particularly useful for optical fiber communications; they are relatively easy to fabricate and less costly than conventional, larger gas lasers; and they have a compact size which is useful in many applications including optical fiber communications, printing and medical treatments.
One example of a particular type of semiconductor diode laser is the broad area diode laser. The "broad area" refers to the junction plane from whence the laser radiation originates. Most broad area semiconductor lasers comprise a so-called "stripe" geometry which provides several advantages. First, there is improved response time due to small junction capacitance. Further, the thin active layer which is the area wherein the laser radiation is generated and confined, contributes to a smaller cross sectional area. This reduces the operating current, which is necessary for sustained operation of the laser, and also reduces the threshold current, which is the current required to induce a laser device to commence lasing action.
However, the laws of diffraction dictate that beam divergence, which is not desirable, will greatly increase with decreasing aperture size. Yet, most applications require a small beam with maximum power in the smallest area possible. Increasing the width of the aperture does not help, because not only does it reduce the power per area of the emitted beam, it has been demonstrated that the modal characteristics are significantly degraded as the aperture width is increased past a certain point. As this width is increased, the mode degrades from a single, good quality Guassian intensity profile, to several filaments (hot spots) of the beam dispersed or spread over the lateral dimension of the beam.
Another example of a semiconductor laser is a diode laser bar. Attempts have been made to increase the power of semiconductor lasers by combining multiple diode lasers into what is termed a "laser array". The advantage of placing individual diode lasers into an array is that the overall output power can be increased by phase locking several diode lasers together such that they operate as a single source. Yet, even though the power does increase when multiple lasers are combined to produce multiple beams, the quality is extremely poor. In turn, users are force to spend increased time and money in attempting to alleviate the poor laser quality of the laser arrays, with less than ideal results.
Over the last decade there has been a tremendous amount of research effort spend in designing and fabricating high power arrays with adequate modal control and degree of coherence. Commercially available diode laser arrays have been available for the last few years which utilize stacked configurations of bars of laser diodes which lie in the grooves of a planar substrate containing a heat sink for the device. These stacked diode bars use a technology which is built upon "rack and stack" configurations. See, e.g., U.S. Pat. No 5,311,535 and No. 5,526,373.
Yet, the use of diode laser bars in this stacked design has a number of disadvantages. The emitted laser beams from laser diode arrays experience significant divergence. This problem is addressed in U.S. Pat. No. 5,311,535 and No. 5,668,825. Specifically, the individual lenses are placed at a predetermined distance with respect to each diode laser. The radiation emitted from each diode laser passes through a lens which collimates the laser beam. Such a system requires the fabrication of multiple of microlenses and the accurate placement of each, which complicates the manufacturing process thereby raising the overall cost of the system. In addition, any misalignments in the placement of the lenses greatly reduces the efficiency of the system, yet adjustments in alignment are extremely difficult. This system at its best is able to convert electrical power into optical power at an efficiency of about 50 percent.
U.S. Pat. No. 5,333,077 suggests an alternative solution to the problem of the diverging emission which comprises a combination of aperture filling and geometrical transformation, and requires arrays of lenses. The lenses are diffractive lenses, the fabrication of which, with the appropriate profiles, is difficult, time consuming, and costly.